Since the last decade of the twentieth century, there has been increasing interest in building cameras that utilize the shortest wavelengths of the radio frequency (RF) portion of the electromagnetic spectrum that still penetrate materials. The range of interest typically starts at the “W band” (about 75 GHz to about 110 GHz) and extends into higher terahertz frequencies. The general concept is to perform imaging to detect and recognize objects of interest concealed behind cover materials such as clothing. This is passive imaging in that natural radiation from a scene is used to perceive differences in the apparent temperatures of objects.
Early on, heterodyne receiver channels performing down-conversion were scanned to create an image. One advance came with the availability of low noise amplifiers (LNAs) operating in the W band. This simplified the design and matched a window in the atmosphere, but the channels were still scanned to create an image. These imaging systems had limited utility in that they often were bulky, required high power, and were complicated.
Non-scanning two-dimensional staring arrays of antenna-coupled micro-bolometers were briefly considered for passive imaging in the atmospheric window at the W band. However, the bolometers were deemed too slow and insensitive to be useful. Some heterodyne imaging systems have also been operated at higher frequencies, but low noise amplifiers with high gains and low noise figures have not been readily available above about 100 GHz.